The invention relates to a sailboat with a keel board that is inclinable sideways.
Known similar boats include:
a hull defining a longitudinal direction which extends between a stern portion and a stem portion;
a keel board which includes a blade which projects beneath the hull to reduce sideways movement of the boat;
a boomed mast on the hull, for carrying sail so that the action of the wind on the sail(s), in conjunction with the action of the water on the blade makes the boat move; and
means for inclining the keel board to allow the blade to incline sideways due to the thrust of the water by rotating through a limited angle about at least one inclination axis which is substantially parallel to the keel plane and above it, so that the transversal component of the movement of the boat generates an upward lift due to the action of the water on the blade.
Such limited inclination of the blade of the keel board provides, in addition to the usual leeway-resisting force generated by the pressure of the water on the leeward surface of the blade, a hydrodynamic upward lift force which reduces the apparent weight of the boat and hence its submerged volume and its hydrodynamic drag.
In the case of a boat having two keel boards, one disposed to port and the other to starboard, the lift of the leeward keel board may be arranged to be greater than that of the windward keel board to increase the righting couple which opposes the tendency of the boat to heel. Such a disposition is illustrated in FIG. 11 of U.S. Pat. No. 3,179,078 (Popkin).
These known dispositions make it possible to obtain hydrodynamic lift only to the extent to which the boat makes leeway due to the action of the lateral component of the thrust of the wind on the sail, i.e. to which its movement has a sideways component. The direction of the above movement is therefore not the direction which generates the least hydrodynamic drag, i.e. the least resistance to the forward motion of the submerged portion of the hull. This is particularly true when the hull is more or less triangularly shaped, with tapered bows and a flat bottom which is wide at the stern. Indeed, in that case the heel of the boat modifies the form of the submerged portion of the hull so that the axis thereof turns about the stem as the boat heels, i.e. the direction of least resistance turns to windward, thereby increasing the angle between the direction of least resistance and the direction in which the boat is moving.
Further, the figures of published German patent application No. 24 60 479 entitled "An inclinable keel for a pleasure boat" (Burmester) illustrate a keel which is inclinable by rotation about an axis of inclination in a vertical longitudinal plane of the boat and inclined with respect to the horizontal rising towards the stern. Such a disposition is complex and does not make it possible to remedy the aforementioned drawback if the blade of the keel board is to provide lift.